1. Field of the Invention
This invention relates to a self-commutated inverter of the kind having terminals for connection to d.c. voltage source means, two main branches connected in series with each other between two of said terminals, each main branch comprising first diode means, a main thyristor connected in anti-parallel with the first diode means, and a commutating inductor connected in series with the main thyristor and the first diode means, and turn-off circuit means connected between a point of connection arranged between the two main branches and at least one of said terminals, the turn-off circuit means comprising commutating capacitor means and thyristor means, controllable in both directions, connected to said commutating capacitor means.
2. Description of Prior Art
A prior art self-commutated inverter of the kind referred to is shown in FIG. 1. The inverter has three terminals A, B and C, the terminals A and B being intended for connection to the positive and negative poles, respectively, of a d.c. voltage source (shown in FIG. 1 as two series-connected d.c. voltage sources U.sub.U and U.sub.L, each having the voltage (U.sub.D /2), and the terminal C being intended for connection to a centre tap of the d.c. voltage source. The two series-connected main branches are connected between the terminals A and B, a first main branch comprising the commutating inductor L.sub.DU connected in series with the anti-parallel-connected main thyristor HU and a diode DU, and a second main branch comprising the commutating inductor L.sub.DL connected in series with the anti-parallel-connected main thyristor HL and a diode DL. The main thyristor and the diode in each main branch may consist of the same component and may possibly be integrated, i.e. formed in one and the same body of semiconducting material. The point of connection F of the two main branches constitutes a phase terminal of the inverter at which a pulsating potential generated by the inverter is obtained. FIG. 1 shows a load object L connected between the point of connection F and the terminal C for supplying the load object with alternating voltage. Alternatively, for example, two or three phase groups (each one designed according to FIG. 1) may be arranged to operate with a mutual phase displacement of 180.degree. and 120.degree., respectively, and a single-phase or three-phase load object or an a.c. voltage network can then be connected to the phase terminal of the phase groups.
A turn-off circuit is connected between the point of connection F and the center tap of the d.c. voltage source. The turn-off circuit consists of two anti-parallel-connected turn-off thyristors SU and SL, a commutating capacitor C.sub.K and a commutating inductor L.sub.K. The two anti-parallel-connected turn-off thyristors SU and SL may, of course, be replaced by a bidirectional thyristor (triac) or by another component or connection which is controllable in both directions. The commutating inductor L.sub.K may be omitted and the entire commutating inductance be formed by the inductors L.sub.DU and L.sub.DL, each having the inductance L.sub.D.
The mode of operation of the inverter shown in FIG. 1 will be clear from the schematic curves shown in FIG. 2, which are valid for no load operation and show the commutating capacitor voltage U.sub.C, the turn-off current i.sub.S and the voltage U.sub.LD across the commutating inductor L.sub.DU. To start with the thyristor HU is conducting and the commutating capacitor C.sub.K is charged to the voltage U.sub.C =U.sub.D. At t=t.sub.1, the turn-off thyristor SU is ignited, the main thyristor HU is turned off, and the oscillating circuit L.sub.K -C.sub.K -L.sub.DU completes half a cycle of an oscillation. The oscillating current flows through the components L.sub.K, C.sub.K, SU, DU, L.sub.DU and U.sub.U. The voltage U.sub.LD across the inductor L.sub.DU has the amplitude ##EQU1## and is thus determined by the inductances of the commutating inductors.
At t=t.sub.2, the thyristor HL is ignited and the oscillating circuit L.sub.K -C.sub.K -L.sub.DL performs half a cycle of an oscillation. At t=t.sub.3, the oscillation is completed and the commutating capacitor C.sub.K has the voltage U.sub.C =-U.sub.D.
At t=t.sub.4, a new commutation is started by ignition of the thyristor SL and thereafter, at t=t.sub.5, the thyristor HU is ignited. At t=t.sub.6, the commutation is completed.
The delay between the ignition of a turn-off thyristor and the subsequent ignition of a main thyristor is designated t.sub.d in FIG. 2.
In the inverter of FIG. 1, the final value of the commutating capacitor voltage U.sub.C after completed commutation is largely independent of the magnitude of the load current i.sub.B of the inverter if the time delay t.sub.d is given a suitable constant value. This type of inverter is normally stable, and particular circuits for feedback of commutating power are not required and are normally not provided. However, the inverter connection has a drawback in that the final capacitor voltage easily becomes asymmetric, i.e. assumes a high value (U.sub.C &gt;U.sub.D) after every second commutation and a low value (U.sub.C &lt;U.sub.D) after the intermediate commutations. This condition is shown in FIG. 3 and may result in the turn-off capacitor becoming so low, during the half cycles when U.sub.C is negative, that the load current cannot be commutated. The only mechanism that controls the capacitor voltage towards symmetry, during no load operation, is the oscillation losses of the inverter connection which, especially in connection with high power inverters, must be held low. The control towards symmetry therefore often becomes so weak that the commutating capacity is lost. Asymmetry of the commutating capacitor voltage may be caused by pulsations of the voltage of the d.c. voltage source, by the load current or by asymmetries in the circuit design. For example, in inverters for higher power, pulsations of the intermediate link voltage are difficult to avoid. This is especially true when supplying the intermediate link from a single-phase network, for example during vehicle operation. During each commutation, the capacitor voltage is easily displaced a certain amount in a direction towards asymmetry. These displacements are accumulated and the result is that the capacitor voltage rapidly becomes so asymmetric that the commutating capacity is lost, resulting in breakdown of operation.
The present invention aims to provide a self-commutated inverter of the kind referred to, in which the commutating capacitor voltage under all operating conditions is well-defined and substantially symmetric, and in which, therefore, the commutating capacity of the inverter, and therefore its operability, is ensured.